This project is based on the observation by the principal investigator that when peritoneal exudate cells (PEC) from tuberculin sensitive animals are cultured in a conical culture tube in the presence of PPD, amplified titers of high and lower molecular weight lymyphokines are released in the culture supernatant. It has also been found that the low molecular weight lymphokine stimulates production of the high molecular weight lymhokine by nonsensitive PEC in the presence but not in the absence of PPD. This basic observation will now be applied to a study of immunity to infection with Listeria monocytogenes as measured by killing of Listeria by guinea pig macrophages. It is proposed to compare the killing of Listeria by peritoneal macrophages derived from Listeria-sensitive and nonsensitive guinea pigs. If the PEC from the sensitive animal kill more effectively than low and high molecular weight lymphokines derived from the PEC of such sensitive animals will be applied to nonsensitive animals to see if they can be transformed so as to kill more efficiently.